Molecular orientation-dependent photochemical behavior

of porphyrin derivative coordinated gold clusters

(ICR. Kyoto Univ., Tokushima Univ.) EGUCHI, Daichi; SAKAMOTO,

Masanori; FURUBE, Akihiro; TERANISHI, Toshiharu

[Introduction]

Organic-inorganic hybrid nanomaterials have received considerable attentions due to their wide

range of applications, such as photo-energy conversion, sensing, and catalysis. Elucidation of

photochemical behavior at the organic/inorganic nano-interface would enhance the efficiency and

functionality of these applications. One of the promising strategies to elucidate the photochemical

behavior at the interface is the precious control over molecular orientation on nanomaterial. In this

study, we designed and synthesized organic ligands, which coordinate on a gold clusters (AuCs) with

different orientation, and elucidated their molecular orientation-dependent photochemical behavior by

transient absorption (TA) measurements.

[Experimental]

We designed the porphyrin derivative (SC1P), which contains four acetylthio groups facing in the

same direction to the porphyrin ring to bind to the AuC in face-coordination fashion (Fig. 1a). SC1P

was synthesized from phthalaldehyde via 5 steps (Fig. 1b). m4SC1P with an acetylthio group, which

binds to the AuC in perpendicular-coordination fashion, was synthesized from 4-

(diethoxymethyl)benzaldehyde via 5 steps (Fig. 1c, d). Structural characterization of these compounds

were carried out by NMR measurement, Mass spectrometry and single X-ray diffraction analysis. The

AuCs coordinated by the SC1P (SC1P-AuC) or m4SC1P (m4SC1P-AuC) were synthesized by the

reduction of Au(Ⅲ) ions in the presence of porphyrin derivatives. The obtained AuCs were purified by

gel permeation chromatography.

Figure 1. Schematic illustrations of coordination fashions of (a) SC1P and (c) m4SC1P on a AuC. Chemical structures of (b) SC1P and (d) m4SC1P.

1P030

[Results and Discussion]

The sizes of obtained SC1P-AuCs and m4SC1P-AuCs were

1.2 ± 0.1 nm and 1.3 ± 0.1 nm, respectively from transmission

electron microscope (TEM) (Fig. 2 a, b). The coordination of

porphyrin derivatives on a AuC cause a damping of ε values

and red shifts of Soret band (Fig. 3 a, b). These phenomena

derived from image charge effects and π-metal orbital

coupling [1].

The excited state behaviors of these AuCs were investigated

by TA measurements. TA measurement of m4SC1P-AuC

showed characteristic peaks at ~750 nm, derived from the

radical cations of the porphyrin moieties (Fig. 3 e, f). On the

other hands, TA measurement of SC1P-AuC showed

structureless broadening peak in NIR regions. The spectral

shape was different from that of radical cations or porphyrins

in the singlet excited state. Therefore, we concluded that the exciplex was formed by the excitation of

SC1P-AuC due to the strong interaction at the ground state. (Fig. 3 c, d).

In this study, we synthesized a series of porphyrin-coordinated AuCs with different molecular

orientation, and elucidated that the molecular orientation is a dominant factor to determine the

photochemical behavior at the molecular-metal nano-interface.

[1] D. Tanaka, Y. Inuta, A. Furube, M. Haruta, Y.-G. So, K. Kimoto, I. Hamada, and T.

Teranishi, Chem. Sci., 2014, 5, 2007.

20 nm

20 nm

(b) m4SC1P-AuC : 1.3 ± 0.1 nm

(a) SC1P-AuC : 1.2 ± 0.1 nm

Figure 2. TEM image of (a) SC1P-AuCs and (b) m4SC1P- AuCs.

Figure 3. Absorption spectra of (a) SC1P and SC1P-AuCs, and (b) m4SC1P and m4SC1P-AuCs. Transient absorption spectra of (c) SC1P-AuCs and (e) m4SC1P-AuCs in DMF. (d) Energy diagram of SC1P-AuC and (f) m4SC1P-AuC.